1. Field of the Invention
The invention disclosed herein relates to the field of gas turbines and, in particular, to increasing the efficiency and limiting the emissions of lean premixed gas turbines.
2. Description of the Related Art
Gas turbines are used to generate a significant portion of electricity to the public and industry. It is important for the gas turbines to operate efficiently and with low emissions. Several operating factors can influence efficiency and emissions.
Combustion temperature in a combustion chamber of a gas turbine is one factor that can influence efficiency. Generally, an increase in the combustion temperature results in an increase in efficiency.
The amount of mixing of fuel and air prior to combustion can influence emissions. During lean premixed combustion, if the fuel and air are not properly mixed, then local areas in the combustion chamber can have mixtures that are either richer or leaner than the surrounding mixture. These richer mixtures burn at higher temperatures than the average combustion temperature and create what are known as “hot zones.” The hot zones generally contribute to larger rates of production of nitrous oxides (NOx). Conversely, leaner mixtures burn at temperatures lower than the average combustion temperature. Combustion of the lean mixtures generally can result in the formation of additional carbon monoxide (CO).
The degree of mixing of the fuel and air in the gas turbine is important to controlling emissions. In addition, if the emissions can be held constant with an increase in combustion temperature, then the gas turbine can be operated with increased efficiency.
Fuel and air mixing is generally performed in the combustion chamber or in the mixing section of a fuel nozzle. The fuel nozzle is used to inject fuel into the incoming airstream, provide a mixing region, and then direct the fuel/air mixture into the combustion chamber. In general, the fuel and the air are each provided by a stream. Improvements to the fuel and air mixing have been handled by modifying or adjusting at least one of generation of air swirl, the type of fuel nozzles, and locations of the fuel nozzles relative to the stream of air. For example, the use of vanes in the stream of air is one way to increase swirl. Even with the improvements described above, some degree of uneven mixing still occurs.
Therefore, what are needed are techniques to improve the mixing of fuel and air for combustion in a gas turbine.